Light source is one of the most critical components of an OFN module. The operation of OFN module will rely on light reflection from a tracking surface to determine its tracking motion. Depending on the motion sensor technology, light-emitting diode (LED) and vertical-cavity surface-emitting laser (VCSEL) will be the options of light source. Usually, both LED and VCSEL are designed for room temperature operation. They will have the best light emitting efficiency at room temperature.
Optical power is a critical factor that affects tracking performance. To minimize optical power variation at high and low temperatures, current compensation for the light source will be needed. The temperature characteristic of LED/VCSEL can vary a lot among light sources. To give the best operating condition, a tailor-made compensation curve will be needed for individual light source. In this case, a dedicated memory space will be needed to store the compensation setting for every light source. However, this requirement is not properly addressed in the existing OFN IC, so usually there is only limited memory space left. There is a need to develop a compensation algorithm for temperature compensation with minimal memory usage.
Also, in normally operation, light source will be driven by a pulse signal at different frequencies and duty cycles depending on the tracking status. When no tracking surface is detected, the pulse frequency and duty cycle will be slower. When tracking surface is detected, the pulse frequency and duty cycle will be increased to catch up the motion. Based on this object detection algorithm, the light source needs to be always operating in pulse mode. It is desirable to develop an OFN with a proximity sensor to minimize the operating time of light source, and thereby increasing the life time of the OFN module.
Thus, there is a need to optimize the operating mechanism of OFN modules to make it more reliable for operation at high and low temperatures so that life time of the modules will be improved by minimizing operating time and optimizing the driving current of light source during extreme temperature.
The above description of the background is provided to aid in understanding the optical finger navigation module, but is not admitted to describe or constitute pertinent prior art to the optical finger navigation module, or consider any cited documents as material to the patentability of the claims of the present application.